Roughly 25% of the amount of liquid distributed in extensive conduit networks, for instance networks which convey water or oil under pressure, is lost because of leakage in the system. This is a serious problem with regard to economy and also with regard to the environment.
One method of minimizing these drawbacks is to have at one's disposal an effective and simple auxiliary device by means of which the distribution system can be checked with the intention of discovering a possible leakage as early as possible.
Personnel engaged in finding leaks are accustomed to relying on their own hearing to ascertain the presence of a leak, by listening directly. It is difficult, however, for the human ear to discern sound that has been propagated in a leaking plastic pipe.
It is also difficult to discern the sound of a leak against background noise, for instance against pronounced traffic noise.
Many different technical solutions relating to listening devices for localizing leakages in different types of pipe systems are known within this technical field. The sounds generated by a substance leaking from a pipe or conduit can be listened for directly, or is signal processed in different ways. Various listening methods have been developed, such as the arrangement of electronic amplifiers which have enabled even relatively weak sound to be discerned. These amplifiers must be used together with acoustic sensors which convert the pressure waves into electric signals. In this regard, there have been used microphones which are pressed directly against pipes or fittings in the same way as the ear of the listener is pressed when listening directly. However, it has been found that the best method of discovering leakages, even small leakages, is to listen directly against the liquid or substance concerned. This is best effected with the aid of a hydrophone, which is coupled to the liquid either through a separate hole drilled in the conduits, or by passing the hydrophone to existing fire cocks, fire hydrants or the like, which are opened so that the water is able to rise to the level of the sensing surface of the hydrophone.
These devices cannot normally be used, however, in conduit networks in which the conduits or pipes are made of a plastic material, for instance, since essentially only low acoustic frequencies of the leakage sound are able to propagate over longer distances in the pipes. These sounds cannot be heard by the human ear, not even with the aid of an amplifier and earphones. It is therefore necessary to use electronic measuring apparatus in cases such as these. This means that the person who is specially trained to find leakages and who is accustomed to listen for sound in cast-iron pipes is unable to apply his expertise in deciding whether or not there is a leakage in the modern plastic pipes. This is a very serious drawback, since the human hearing is paramount with regard to distinguishing between different sounds that occur in pipes, i.e. between leakage sounds or traffic sounds.
The acoustic converters used in hydrophones are normally comprised of piezoelectric crystals having a high resonance frequency, e.g. a frequency of 100 kHz. This crystal can be combined with a mechanical element, such as a diaphragm, having a lower resonance frequency.
Examples of such known devices are found in U.S. Pat. Nos. 2,008,934, 4,435,974, 4,236,235, 4,779,246 and 4,810,913.
The device taught by U.S. Pat. No. 4,779,246 includes a diaphragm which has been tensioned in a way to provide the lowest resonance frequency.